1. Field of the Invention
This invention relates to a servo-control apparatus for controlling the velocity of a servomotor. More particularly, the invention relates to a servocontrol apparatus capable of eliminating the influence of a disturbance torque.
2. Description of the Related Art
In velocity control of an AC motor such as an induction motor or synchronous motor, a current command (torque command) conforming to the error between a commanded velocity and actual velocity is generated to make the actual velocity coincide with the commanded velocity. FIG. 7 is a block diagram of a conventional velocity loop, in which numeral 1 denotes an arithmetic unit for calculating a velocity error, which is the difference between a commanded velocity V.sub.CMD and actual velocity V.sub.FB, 2 an integrator having an integration gain k.sub.1 for integrating the output of the arithmetic unit 1, 3 a proportional unit set to a proportional gain k.sub.2 for outputting k.sub.2 .multidot.V.sub.FB, 4 an arithmetic unit for calculating the difference between the outputs of the integrator and proportional unit, 5 a setting unit for setting a torque constant K.sub.t, 6 an arithmetic unit for adding a disturbance torque and the output of the torque-constant setting unit, and 7 a motor. J.sub.m represents motor inertia.
In the velocity loop of FIG. 7, the influence of the disturbance torque appears as a fluctuation in motor velocity and is reflected in the torque command through the integrator 2, having a transfer function of is k.sub.1 /s, and the proportion unit 3, whose proportional gain is k.sub.2. Consequently, even through a fluctuation in velocity is produced by the disturbance, the overall velocity loop acts in a direction to reduce the fluctuation in velocity. The higher the gain of the velocity loop, the greater the degree to which the disturbance is suppressed. However, there is a limit to the gain, for the overall system will fall into an oscillatory state if the velocity loop gain is made too high. In other words, if the disturbance torque is large, the fluctuation in motor velocity cannot be suppressed sufficiently because the degree of suppression cannot be raised. For example, when cutting is performed by a machine tool, problems occur, such as an uneven cut surface.
Accordingly, an object of the present invention is to provide a servo-control apparatus in which the system will not oscillate and which is capable of applying a sufficient torque correction with regard to the disturbance torque, thus making it possible to sufficiently suppress a fluctuation in motor velocity due to a disturbance torque.